Conventional sonic logging devices measure the velocity of critically refracted compressional and shear waves in the formation and date back to the Vogel article "A Seismic Logging Method," GEOPHYSICS, v.17, number 3, July, 1952 and U.S. Pat. No. 2,651,027. There are other treatments of this tech5 nology such as "Elastic Waves in Layered Media"; Ewing, W. M., Jardetzky, W. S. and Press, F., 1957, McGraw-Hill. Also discussed in conventional technology was a "borehole televiewer" (Zemanek, J., et al, 1969 "The Borehole Televiewer: A New Logging Concept for Fracture Location, and "Other Types of Borehole Inspection," J. Petr. Tech., v. 21, pp. 762-774 in 1969) that obtained an acoustic reflectance image of the borehole wall at normal incidence.
Other treatices include Stoll, R. D. and Kan, T. K., 1981, "Reflection of Acoustic Waves at a Water-Sediment Interface," J. ACOUST. SOC. AM., v. 70, no. 1.
None of the prior art approaches however, facilitated measuring formation compressional and shear wave velocity from reflected acoustic signals. Neither did they make possible the calculation of the densities of the respective formations penetrated by the borehole at their respective depths using reflected acoustic signals.
While the prior art has provided method and apparatus for obtaining refracted acoustic signals from which compressional-wave and shear-wave velocity can be calculated, the resolution of these devices is on the order of feet. In contrast, the use of reflected signals by the method described herein can provide resolution on the order of inches. This is accomplished with a continuously moving sonde which is not required to make contact with the borehole wall, thereby facilitating the logging operations.
Particularly, the prior art has not provided a method and apparatus for measuring both compressional and shear wave velocities and providing data from which densities of the subterranean formations could be calculated, both using acoustic energy, without requiring advent to more exotic forms of logging, such as nuclear magnetic resonance logging or gamma ray excitation logging.